The present invention relates to a new electrostatic dissipation/electromagnetic interference shielding element. More particularly, the invention relates to an element having an inorganic substrate with a tin oxide-containing material, preferably an electronically conductive tin oxide-containing material.
In the above-noted applications it would be advantageous to have an electronically conductive tin oxide which is substantially uniform, has high electronic conductivity, and has good chemical properties, e.g., morphology, stability, etc.
A number of techniques may be employed to provide conductive tin oxide coatings on acid resistant substrates. For example, the chemical vapor deposition (CVD) process may be employed. This process comprises contacting a substrate with a vaporous composition comprising a tin component and a dopant-containing material and contacting the contacted substrate with an oxygen-containing vaporous medium at conditions effective to form the doped tin oxide coating on the substrate. Conventionally, the CVD process occurs simultaneously at high temperatures at very short contact times so that tin oxide is initially deposited on the substrate. However tin oxide can form off the substrate resulting in a low reagent capture rate. The CVD process is well known in the art for coating a single flat surface which is maintained in a fixed position during the above-noted contacting steps. The conventional CVD process is an example of a "line-of-sight" process or a "two dimensional" process in which the tin oxide is formed only on that portion of the substrate directly in the path of the tin source as tin oxide is formed on the substrate. Portions of the substrate which are shielded from the tin oxide being formed, e.g., such as pores which extend inwardly from the external surface and substrate layers which are at least partially shielded from the depositing tin oxide by one or more other layers closer to the external substrate surface, do not get uniformly coated, if at all, in a "line-of-sight" process. A particular problem with "line-of-sight" processes is the need to maintain a fixed distance between the tin source and the substrate. Otherwise, tin dioxide can be deposited or formed off the substrate and lost, with a corresponding loss in process and reagent efficiency.
Although the CVD process is useful for coating a single flat surface, for the reasons noted above this process tends to produce non-uniform and/or discontinuous coatings on woven glass fiber mats. Such non uniformities and/or discontinuities are detrimental to the electrical and chemical properties of the coated substrate. A new process, e.g., a "non-line-of-sight" or "three dimensional" process, useful for coating such substrates would be advantageous. As used herein, a "non-line-of-sight" or "three dimensional" process is a process which coats surfaces of a substrate with tin oxide which surfaces would not be directly exposed to vaporous tin oxide-forming compounds being deposited on the external surface of the substrate during the first contacting step. In other words, a "three dimensional" process coats coatable substrate surfaces which are at least partially shielded by other portions of the substrate which are closer to the external surface of the substrate, e.g., the surfaces of the internal fibers of a porous mat of glass fibers.
Although a substantial amount of work has been done, there continues to be a need for a new method for coating substrates with doped tin oxide.